1st Joint Commission 2 and IGFS Meeting
International Symposium on
Gravity, Geoid and Height Systems 2016

September 19-23, 2016
Thessaloniki, Greece

Assessing Mass Transport in the Earth System with the Next Generation Gravity Mission Concept

19/09/2016 | 14:15 | Session 1: New missions and concept performance

Author(s):

Luca Massotti, Roger Haagmans, Christian Siemes and Pierluigi Silvestrin

Abstract

The paper addresses the preparatory studies of future ESA mission concepts devoted to improve our understanding of Earth’s mass transport phenomena observable as temporal variations in the gravity field, at different temporal and spatial scales. These are due to ice sheet and glaciers growth or melting, changes in continental hydrology, sea level change and deep ocean mass dynamics, atmospheric dynamics and solid-earth deformations. The ESA initiatives, started in 2003 with a first study on observation techniques for future solid Earth missions. This was recently continued through several system studies, technology developments and constellation simulations. The activities concentrated for example on propulsion, e.g. tests on miniaturized ion thruster, or distance metrology, e.g. laser interferometry. These activities received precious inputs from the in-flight lesson learnt from ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission and the American-German Gravity Recovery and Climate Experiment (GRACE) mission.

Two parallel studies for the “Assessment of a Next Generation Gravity Mission to Monitor the Variations of the Earth’s Gravity Field” (concisely: NGGM) were performed, one led by Thales Alenia Space (Turin, Italy) and another one by Astrium GmbH (Friedrichshafen, Germany). Both consortia included European universities and academic institutions for scientific support and requirement assessments. Several mission aspects were analyzed, leading to different mission concept features. The preferred mission concepts fitting the at that time defined programmatic boundary conditions have been studied with prioritized science requirements and detailed system designs. In addition to drivers such as tight propulsion requirements and accelerometer calibration issues, resulting in a dominant error source at large scales, technical constraints on power and fuel generally dictate the choice of orbit. Thus, for each considered constellation of satellite pairs, the different interactions between drag-free and “loose” formation control have been analyzed together with the design of the relative attitude control that is necessary to ensure the inter-satellite laser link all along the mission length.

Since then, several complementary GSP (General Study Programme) and TRP (Basic Technology Research Programme) studies were initiated and some are currently running, notably:

  • Next Generation Gravity Mission: AOCS Solutions and Technologies, with the objective to define and evaluate the mission-critical Attitude and Orbit Control System (AOCS) solutions, to identify the critical technologies and to assess their feasibility and the design drivers.
  • Miniaturized Gridded Ion Engine (GIE) Breadboarding and Testing: the ongoing activity aims at developing a miniaturized GIE with increased thrust level, with thrust ranging from 50 μN to 2.5 mN.
    Consolidation of the micro-PIM Field Emission Thruster design for NGGM: In particular the study aims to demonstrate that the mN-FEEP thruster is an excellent candidate for the lateral thrusting on NGGM.
  • High-Stability Laser with Fibre Amplifier and Laser Stabilisation Unit for Interferometric Earth Gravity Measurements: The objective of this development is the manufacturing of an elegant breadboard (EBB) of a high stability laser (HSL) reaching TRL 6 at subsystem level in 2016.
  • Assessment of Satellite Constellations for Monitoring the Variations in Earth's Gravity Field: aimed at the optimization of “Bender-type” constellations of two pairs of satellites for the retrieval of the time-variable gravity field to monitor mass distribution and transport. The study focused on the reduction of temporal and spatial aliasing by optimizing the design of the constellation and by explorating more advanced methods for the time-variable gravity field retrieval. The latest results of this scientific study will be presented in parallel sessions at the LPS 2016.

At the time of writing this abstract, a new system study – named “ Consolidation of the system concept for the Next Generation Gravity Mission” – is in preparation: the purpose of this technical work is to focus on the design of a single satellite pair of the NGGM constellation that should have the flexibility to operate successfully both in near-polar or inclined orbits.

Meanwhile, the user community, also via a recent resolution of the IUGG (International Union of Geodesy and Geophysics), formally invited the space agencies to extend the gravity field science and application monitoring with a concerted international effort. In the framework of a cooperation on future EO technologies and missions, ESA and NASA therefore established an Interagency Gravity Science Working Group of experts tasked to define the overall objectives and the observation requirements for a future constellation with better performance than that of a concept elaborated by a single agency – in fact, a performance better than “the sum of the parts” when these were not jointly defined. The latest results concerning the preferred satellite architectures and constellations, payload design and estimated science performance will be presented as well as remaining open issues for future concepts. Attention will also be given to the ongoing ESA-NASA inter-agency cooperation to align preparatory work on synergistic mission concepts beyond GRACE Follow-On.

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